What a Microfilter doesA microfilter performs two main tasks. First, it prevents
high frequency noise from appearing at the telephone and potentially decreasing
voice quality. Second, it prevents the telephone equipment from interfering
with the modem by preventing harmonics, overtones and changes in impedance
caused by the telephone equipment.

What's InsideNot all filters are equal. Below are some pictures
of the components inside some popular UK microfilters.

The most advanced UK microfilter. The XF-1e uses transistors
to actively remove the ADSL signal from the telephone equipment.
The filter contains the most components used in any UK filter, the
extra components are used to refine the frequency separation.

An interesting filter, the only filter other
than the XF-1e we have found that has a transistor based
design. looking closer at the design it seems to be overly complicated
and didn't achive the accurate seperation required. This filter
has a 1uF ring capacitor that may cause ringing problems with
some
phones.

This is the face plate that can be fitted to the standard
BT NTE-5 master socket. Designed to run all your phones from one
central filter. To support the full 4 REN it has a large 1.8uF ring
capacitor.

This is the face plate as installed by a BT engineer.
Designed to run all your phones from one central filter. To support
the full 4 REN it has a large 1.8uF ring capacitor (the yellow one
at the top).

The
XTF pre-filtered telephone sockets are available in 62 & 80mm versions.
Contains XF-1e filtering, using transistors to actively remove the ADSL
signal from the telephone equipment. The filter
contains the most components used in any UK filter, the extra components
are used to refine the frequency separation.

Safecom SAMF-3000

3 Ferrite cores
8 Capacitors
4 Resistors
2 Chokes
1 Spark Gap

Quite a good filtered telephone socket considering
it's low price. With filtered phone outlet and unfiltered RJ11
outlet for ADSL. Can be used to run all your phones from one central
filter. To support the full
4 REN
it
has a large
1.8uF
ring capacitor
(the
red
one on the top left).

Not exactly a socket but a hardwired filter in
a standard size box that is often used for alarm systems. The circuit
is a simplified
version of the Pressac
ZUK-BT in-line filter near the top of the page here.

Excelsus are a long established name in ADSL filters
and this filter has a reasonably good circuit design for a small plug-in
filter however it falls flat on it's face due to the fact that
the filter does not filter the ring line or provide it's own ring
signal.

This filter has the ring connection incorrectly wired
straight through from the unfiltered line. The ring signal from the
line should not be connected as the filter should provide it's own
ring
signal
using
a ring capacitor.
This filter will cause serious problems on telephones that make use
of the separate ring signal. It does not come anywhere near meeting
the BT SIN 346 standard and was designed for the Hong Kong market.

ASL 2019

1 Ferrite core (2 coils)
1 Diode
2 Capacitors
2 Resistors

Reported to be sold by ebuyer at the moment. ebuyer
were selling the solwise v1 design untill recently, but places like
ebuyer tend to have whatever they can get the cheapest at any given
time and at the moment it seems to be this one.

This filter has the ring connection incorrectly wired
(green wire). The ring signal from the line should not be connected
as the filter should provide it's own ring signal using a ring capacitor.
You can improve the results by cutting the green wire if you are
feeling brave.Claimed to be BT SIN 346 Compliant but
clearly not !

Unknown Brand - Falsly sold as D-Link DSL-20MF by some retailers
!

1 Ferrite core (2 coils)
1 Diode
2 Capacitors
2 Resistors

Surprisingly this unknown brand filter has been sold
as a D-link filter. This filter has a significant design flaw. The
ring signal from the line should not be connected to the filter.
The filter should provide it's own ring signal using a ring capacitor.
This could signal the beginning of a worrying trend of retailers
not delivering what you pay for. The real DSL-20MF is an external
box with small wire design, not a plug-in module like this one.
If you recieved this filter when ordering D-Link filters demand
that they are replaced.

Circuit Diagram of the YPL-002
Microfilter

Circuit Diagram of the YCL MF603UK
Microfilter

About the star rating.

Because even though cheap filters are not 100% they are
often adequate for many applications. As a supplier of ADSL equipment
we have received many filters from manufacturers claiming to be UK compatible
but almost all of them fall short of that claim.

We have developed a star rating to help people better understand
the capabilities of each filter as follows.

The worst are US types that often have significant design errors such
as no ring capacitor. They can just about work with basic phones that
you don't need to ring but best avoided as they are likely to cause serious
degradation of line quality. Fortunately filters in this category are
becoming hard to find.

Low cost filters usually modified US or Chinese design with a ring capacitor
added. Usually this capacitor is inadequate but will work with some basic
phones.

Filters made for the UK with a little more thought gone in to them than
the other 2 but still using a minimum number of components to keep costs
down, this can cause variations in performance depending on equipment
in use and can have problems with difficult to drive loads like some DECT
phones. Filters in this category are now the most common filter sold by
most retailers, also sold on eBay by several traders.

These filters are designed for the UK market, use good quality components
and have a high capacity ring capacitor. These have been well tested by
the specialist ADSL outlets that sell them and may well be tested and
approved by BT using their SIN 346 specifications. They generally work
well with a majority of equipment including DECT phones. They still generally
have problems with a small amount of equipment such as high powered ringers,
caller display units and PABX systems.

Maximum compatibility & performance. If your equipment does not work
with a 5 star rated filter then you are unlikely to get it working as
you almost certainly have a fault with the equipment or line. Currently
the XF-1e Professional microfilter is the only one that we have given
a 5 star rating to. You would be forgiven for thinking that's just because
it's our filter, but the truth is that we have given it this rating because
it really is unbeaten for compatibility. With the XF-1e we are setting
new standards for compatibility that we hope others will follow and even
exceed in the future. We would be happy to see this category grow and
the others shrink as improvements are made to filter design.

Half stars are given where a filter performs better than
others in it's category but not quite enough to meet the criteria for
the next category. The easy way to use the star guide is that the lower
the number of stars the higher the probability of problems caused by the
filter being inadequate.